This invention relates to the formation of fusible coatings or structures (e.g., polymer or polymer composite coatings, or reinforced polymer coatings, as well as polymer, or reinforced polymer structures) via a thermal spray process. In particular, the invention relates to formation of these coatings or structures using a flameless thermal spray process.
Polymer thermal spray systems have been used for depositing thermoplastic, thermoset and radiation-curable materials onto substrates for several decades. Generally, a flame or plasma is projected from a tube and a stream of the polymer (or other fusible powder) is introduced into the projected flame or plasma. In the flame or plasma, the polymer particles are heated and melted, and when they impinge on the surface, they “splat” (flatten) and fuse together to form a more or less continuous coating.
A commercial thermal spray system that uses a gas flame that is available from Xiom, Inc. (Babylon, N.Y.) is the XIOM 5000 Scorpion and the XIOM 1000. Similar units are or have been available from Alamo Supply Company, LTD (Houston, Tex.), model number ASC PG-550, Plastic Flamecoat Systems, Inc (Big Springs, Tex.), Applied Polymer Systems (Boynton Beach, Fla.), and Sulzer Metco, Winterthur, AG, Switzerland.
One major difficulty is the flame or plasma itself. Many polymers degrade in a flame, even when exposed to it for a very short time. In addition, the flame impinging on the target surface can degrade that surface as well. The degradation resulting from oxidation and/or breaking of polymer bonds causes reduction in the physical properties of polymers, as well as the potential inclusion of defects in polymer coatings that are deleterious to their performance or appearance, e.g., burned particles, carbon inclusions, yellowing and discoloration, pinhole defects. As such it is important that thermal methods used to melt, deposit and process polymers as coatings and/or structures do not subject the polymers to harsh oxidation, radical or reactive ion environments that cause charring, or to high temperatures that break or weaken polymer bonds. Thus, the application of polymers for many applications should use the minimum amount of heat during the process in order to preserve the physical properties of the polymers being applied.
The degradation processes related to the use of flames are related to the nature of flames. Flames induce combustion which is a highly exothermic self-sustaining oxidation reaction that in turn produces high temperature flames. The temperature of a flame is primarily dependent on the type of fuel involved in the combustion process and typically ranges from about 1,400 degrees Centigrade (° C.) for a candle to more than 3,000° C. for an oxyacetylene torch. The high temperature of the flame tears apart the vaporized fuel molecules, forming various combustion products and free radicals which react with each other and with the oxidizer involved in the reaction. The high energy of the flame also excites the electrons in some of the transient reaction intermediates such as CH and C2, which results in the emission of visible light as these substances release their excess energy.
The visible part of a flame is extremely rich with chemical reactions and intermediate species, mostly radicals. For example, it has been reported that combustion of natural gas can be modeled using 53 species and 325 elementary reactions. As the temperature decreases beyond the visible part of a flame, most of the highly reactive radicals and ions recombine into less reactive combustion products with stable atomic structures such as CO, CO2, H2O, etc., (for a hydrocarbon based combustion).
During a combustion based polymer thermal spray process, it is desirable to avoid exposure of polymer surfaces to the visible, i.e., highly reactive, part of a flame. The visible part of a flame contains excited species of O, NO, OH, and NH ions. These free radicals can attract hydrogen from polymer surfaces causing surface oxidation and generation of polar oxygenated groups. The oxidation proceeds by a free radical mechanism accompanied by chain scission. Chain scission occurs during free radical propagation yielding polar and mobile scission products. For example, it has been found that hydroxyl, carbonyl, carboxyl, and amide groups are typically found on flame-treated polyethylene. It has been reported that a flame contact period of only 0.01-0.1 second is sufficient to oxidize, i.e., damage, polymer surface to a depth of about 40-90 Angstroms (A).
For the reasons described above, flame treatment is often used commercially to make polyolefines, polyacetales, and poly(ether terephthalate) printable and bondable by introducing polar oxygenated groups into the thin surface layer. However, during a combustion based polymer thermal spray process, coating unit building blocks are individual powder particles. If the surface of each polymer particle is exposed to an open flame and oxidized, the resulting coating will have an abundance of highly distractive free radicals and mobile scission products distributed throughout the entire coating volume. Such coatings can have significantly shortened service life due to reduced mechanical and barrier properties and an accelerated degradation kinetic. The negative effect of the visible flame on thermally sprayed polymer can be significantly minimized by utilizing thermal energy of lower temperature hot gas occurring downstream and beyond the visible flame zone.
One solution to these aforementioned problems is the use of a flameless system wherein a stream of gas is heated electrically, and the polymer and other particles are introduced into the hot gas stream. This greatly reduces burning and degradation of the coating particles and the substrate. A commercial system using this method is disclosed in PCT patent application publication WO/2008/127227, and is commercially available as the Resodyn PTS 1000 system (Resodyn Corporation, Butte, Mont.). Systems to date have used resistive heaters powered by electricity. These systems are less energy efficient than combustible fuel systems, but they avoid exposing the sprayable material to a reactive flame, or plasma, thereby yielding superior results.
For all of its advantages, the use of an electrically resistive heater to transfer heat and melt the polymer or other fusible particles has limitations. In that higher power is required in order to increase the spray output rate, such devices utilize robust sources of electricity and relatively thick copper power cables to avoid undue losses. As a result, while flameless sprayers may achieve. improved results, such sprayers are heavier and less portable than their traditional flame- and plasma-based counterparts.
What is needed is a portable, easy to wield system that can spray large quantities of melted, or partially melted, fusible polymer powder particles and/or other materials onto a substrate, which poses reduced risk degradation of such materials while achieving the necessary heat required for melting and fusing the polymer powders and other materials onto a substrate.
The background art is characterized by U.S. Pat. Nos. 3,801,020; 3,958,758; 4,416,421; 4,694,990; 5,236,327; 5,285,967; 5,503,872; 5,932,293 and 7,216,814; by U.S. Patent Applications No. US2006/166153 and US2009/095823; and by International Patent Application No. PCT/US2007/009021; the disclosures of which patents and patent applications are incorporated by reference as if fully set forth herein.